Do programs such as JROTC and BSA produce young men and women that are more likely to succeed in the military?

I think it helps a little bit, but it is more the character of the person. The best sailor I ever had working for me, was a high school dropout with a GED. He had no time for extracurriculars, as he worked odd jobs to help his Mom. I was in HS JROTC and was an Eagle Scout. Never really found that it helped me out that much. My 2 cents...

It certainly helped me. It teaches you about leadership, attention to detail, responsibility, and drill and ceremonies.

I think it helps to a certain extent if the student takes it seriously. I enrolled in JROTC in high school. The lesson plans were directly out of FM 22-100 and the instructor was a grizzled old retired MSG from the Vietnam era. He was a no-nonsense kind of guy and added many of his own experiences his career and tours of duty.
When I enlisted in the Army myself, a few years later, I entered with some knowledge that other enlisted without any prior exposure had. Whilw that knowledge didn't garner any special treatment, nor should it have, I did have the benefit of knowing what I was in for and why. Honestly though, being an Army brat and having a dad who was an drill instructor, literally, may have had a little something to do with my preparations too. Happy Fathers Day, Dad.